This invention relates to microphone assemblies and, in particular, to microphone assemblies having a built-in audio amplifier.
The very large number of radio transmitters now operating in any given band creates a constant background noise level making communication difficult.
Many techniques have been developed for enhancing the intelligibility of speech transmitted, for example, by a radio transceiver. The simplest form of these techniques is clipping, wherein the average modulation is increased by removing the amplitude peaks found in speech. Some difficulties associated with this technique include increased distortion at high limiting levels and the generation of high order products which further reduce intelligibility unless careful filtering is used. More advanced techniques include fast attack, slow release, automatic gain control amplifiers commonly called audio commpressors. Some compressors have dynamic ranges in excess of 60dB with very low attendant distortion. However, it has been shown that unless some form of frequency shaping precedes the clipping or automatic gain control stage, the average transmitted power may actually increase with little improvement in intelligibility. This phenomenon is a result of the energy frequency distribution of human speech.
Amplified communication microphones are widely utilized to increase the average "talk power" but they create other problems. These microphones are generally equipped with simple nonautomatic gain control transistor amplifiers with accessible gain controls and, in order to attain a high average voice modulation level, the transmitter is over modulated on the peaks of the voice signal. This over-modulation causes distortion, adjacent channel splatter and, in effect, reduces intelligibility of the transmitted signal.
Furthermore, the circuits are disposed within the microphone case and are powered by replaceable batteries. To obtain reasonable battery life in these circuits in microphones, mercury batteries may be used as the primary power source. These batteries are relatively expensive and replacements are not generally available. When used, these batteries have a useful life which is a function of their Amp-hour capacity which, in turn, is a function of their physical size. As a result, some commercial amplified microphones are uncomfortably large and awkward to handle since the batteries and the audio compressor are mounted within the microphone case. The service life of these batteries is, of course, a function of their usage schedule and, since usage schedules are nearly impossible to predict, these batteries must be frequently replaced to assure that power is always available.
The present invention does away with the need for having bulky power supplies within the microphone case and also the necessity of frequently replacing the power supply such as a battery.
A microphone assembly constructed in accordance with the instant invention includes a transducer means for converting an audio signal to an electrical signal, an amplifying means for amplifying the electrical signal, a rechargeable power supply means for supplying power to the amplifying means, connection means for electrical connection to a radio transceiver to transmit electrical signals between the microphone assembly and the transceiver, and switching means for placing the power supply means in electrical contact with the amplifying means and the amplifying means in electrical contact with the connection means during a first or transmit mode and for placing the power supply means in electrical contact with the charging means and the charging means in electrical contact with the connection means during a second or receive mode.